Deep-bed extractors

For example, if there is a deep bed extractor operating on 0.38-mm-thick soybean flakes with a 3.0-m bed depth and a forward velocity of 0.3 m/min, the distance that the miscella collection receptacle needs to follow the washing nozzle can be calculated as follows ... 

All rotary extractors employ deep beds of solids to be extracted, sometimes as much as two or more meters deep. Hence, rotary extractors are also termed deep-bed extractors. There are several mechanical variations of these extractors. Two variations have already been mentioned. Another variation is where the individual cells have hinged bottoms that swing open whenever the cell is to be emptied. In this variation, the solids slide against a common one-piece bottom that has an opening to allow solids to fall through when extraction is completed. Termed sliding plate. 

Figure 22. Stationary deep-bed extractor (Courtesy of The French Oil Mill Machinery Co., Pigua, Ohio).

If a deep-bed rotary extractor is run so that the solvent/miscella makes only one pass through each stage, then preparation should be monitored to ensure that miscella does not rush through any stage without spreading across the surface or that no stage floods. Usually, on a deep-bed extractor, flooding is more likely to... 

One school of thought favors a shallow (20-30 cm) bed of cake crumbles (or flakes in the case of soybeans or sunflowers) whereas another school believes that deep beds are not an obstacle to good extraction, are more compact and provide more capacity per unit of building volume. Provided the bed permits an adequate percolation rate the deep bed extractor seems to be equally as successful as the shallow bed type. 

Hardly any batch-type oilseed extractors remain, and modern solvent extractors mainly are of two basic designs. In shallow bed-type extractors, a 0.5- to 1-meter-thick layer of collets or flakes is pulled across a linear screen (Fig. 8.6), or conveyed on a woven belt, and repeatedly percolated with solvent. Deep bed extractors mainly are constructed as carousels with pie-shaped cells (baskets), that are alternately filled (2-3 meters deep), extracted, and unloaded. In some designs, the baskets rotate between the loading, extraction, drainage, and unloading stations (Fig. 8.7) in others, the baskets are stationary with the various stations revolving (Fig. 8.8). Solvent flow always is countercurrent to the direction of... 

The extractor can run as a deep bed or as a shallow bed. Usually, the bed depth is intermediate. There are several miscella stages and a hnal hexane wash (Figure 24). Each miscella sump pump recirculates miscella back onto the same stage the miscella came from, except for the hnal full miscella. Recirculation hows can be... 

Carousel extractors suitable for extracting phytochemicals from plant materials have also been adopted from equipment used extensively in the oilseed extraction industry. The carousel extractor consists of a sectioned cylinder, as shown in Figure 11.14. Raw material is slurried with extract and loaded into the initial cell. Solvent is added to each cell countercurrently, collecting at the bottom of each cell, and is then pumped to the next cell. Either flooded (deep bed) or trickle (shallow bed) operation is possible depending on the solvent flow rate to each cell and equipment design. After draining the solvent in the last cell, the marc is discharged from the cell and conveyed to external solvent removal equipment. Concentrated extract is collected from the raw material feed cell. 

Solvent (hexane) extraction of soybeans is a diffusion process achieved by immersing solid in solvent or by percolating solvent through a bed of solids. Rotary (deep-bed), horizontal belt, and continuous loop extractors are used for soybeans (Woerfel 1995). Solvent is recovered from the mixture of solvent and extracted oil (miscella) by double-effect evaporator and steam stripping and from flake by a desolventizer-toaster, and is recycled. 

Crossflow extractors are used to extract meny different solutes and can handle a wide variety of solid feeds.11 Some crossflow extractors ate quite laige, for example, 11 m wide and 52 m long and can handle 10,000 tons of solid feed per day. They msy contain up to IS stages. The solid beds are usually between 0.5 and 3,0 m deep, although in the Filtrex exiaclor 0.05 m deep beds are used. 

If a deep bed u used in a reflux extractor the exlractur can be analyzed tike fixed-bed extractors operating with dtiwnftow and yj0 0. In such a case V = Ee. However, if the bed is Hooded and consequently part of the reflux bypasses the bed. V should be based on an Eh that barely results in bed saturation rather than the actual Efe. If the refluxed solvent b not passed throngh a bed but is edded 10 a well-mixed batch of extract and solid and the rate of extract discharge is E . the extractor can be treated like a diflcrtmriel extractor. Reflux extractors in which very short beds are used can also be treated like differential extractors as a mesonablc approximation. 

A second type of extractor widely used on soybeans is the deep-bed (34 m deep), rotary-basket extractor (Fig. 11.13), such as the Reflex extractor marketed by Desmet Ballestra. Countercurrent solvent-to-flake flow is accomplished by rotating the baskets of flaked material while the solvent and miscella sprays, the miscella collection cells, and the marc discharge remain stationary. The bed is divided into cells or baskets to prevent back mixing of oil-lean miscella with oil-rich miscella. The drained marc discharges when the basket rotates to the position above the discharge hopper. 

Fig. 11.13. Depiction of a deep-bed, rotary basket extractor ("Reflex" extractor, Desmet Ballestra North America, Marietta, GA).

Provision of extractor baskets. In some situations it is desirable (or even necessary) to enclose the solids which are to be extracted in a set of baskets which are then placed in the extraction vessel. This procedure is necessary in the case of materials which tend to form compact agglomerates when extracted in a deep bed. Such agglomerates can be virtually impossible to extract and, furthermore, the compressed bed containing them does not flow... 

Fig. 8.7. Cutaway drawing of Rotocell Solvent Extractor. A carousel with deep-bed baskets rotates by a series of collet/flake loading, solvent extraction, drainage, and unloading stations. (By permission of Davy-Dravo, Inc., Pittsburgh, PA.)...

Eckert indicated that the rate of mass transfer decreased rapidly as the residence time of the dispersed-phase droplets in the continuous phase increased [19]. Thus, the act of formation of the dispersed-phase droplets contributes significantly to the overall mass transfer. Therefore, the use of packed beds of 8 to 12 ft depth followed by redispersion tends to minimize column height. Seibert et al. ran tests using two beds of packing, each 1L5 ft deep, in a 16.7- in. ID extractor [14]. With 25 IMTP packing, they found a 40% improvement in the overall mass transfer coefficient when redispersion of the organic phase occurred between the two packed beds. 

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